The present invention concerns spinal instrumentation systems, most particularly for use in fixation of the cervical spine. More particularly, the invention pertains to a plating system for use in the treatment of various spinal pathologies.
As with any bony structure, the spine is subject to various pathologies that compromise its load bearing and support capabilities. The spine is subject to degenerative diseases, the effects of tumors and, of course, fractures and dislocations attributable to physical trauma. Throughout the last century, spinal surgeons have tackled the thorny problems associated with addressing and correcting these pathologies using a wide variety of instrumentation and a broad range of surgical techniques. For many years, the use of elongated rigid plates has been helpful in the stabilization and fixation of the lower spine, most particularly the thoracic and lumbar spine. These same plating techniques have found a tougher road towards acceptance by surgeons specializing in treatment of the cervical spine.
The cervical spine can be approached either anteriorly or posteriorly, depending upon the spinal disorder or pathology to be treated. Many of the well known surgical exposure and fusion techniques of the cervical spine are described in a publication entitled Spinal Instrumentation, edited by Drs. Howard An and Jerome Cotler. This text also describes instrumentation that has been developed in recent years for application to the cervical spine, most frequently from an anterior approach.
The anterior approach to achieving fusion of the cervical spine has become the most popular approach. During the early years of cervical spine fusion, the fusions occurred without internal instrumentation, relying instead upon external corrective measures such as prolonged recumbent traction, the use of halo devices or minerva casts, or other external stabilization. However, with the advent of the elongated plate customized for use in the cervical spine, plating systems have become predominant for this type of spinal surgery.
It has been found that many plate designs allow for a uni-cortically or bi-cortically intrinsically stable implant. It has also been found that fixation plates can be useful in stabilizing the upper or lower cervical spine in traumatic, degenerative, tumorous or infectious processes. Moreover, these plates provide the additional benefit of allowing simultaneous neural decompression with immediate stability.
During the many years of development of cervical plating systems, particularly for the anterior approach, various needs for such a system have been recognized. For instance, the plate must provide strong mechanical fixation that can control movement of each vertebral motion segment in six degrees of freedom. The plate must also be able to withstand axial loading in continuity with each of the three columns of the spine. The plating system must be able to maintain stress levels below the endurance limits of the material, while at the same time exceeding the strength of the anatomic structures or vertebrae to which the plating system is engaged.
Another recognized requirements for a plating system is that the thickness of the plate must be small to lower its prominence, particularly in the smaller spaces of the cervical spine. The screws used to connect the plate to the vertebrae must not loosen over time or back out from the plate. Preferably the plate should be designed to contact the vertebrae for greater stability.
On the other hand, while the plate must satisfy certain mechanical requirements, it must also satisfy certain anatomic and surgical considerations. For example, the cervical plating system must minimize the intrusion into the patient and reduce the trauma to the surrounding soft tissue. In the Spinal Instrumentation text, as well as in other documentation in this field, it is stressed that complications associated with any spinal procedure, and most particularly within the tight confines of cervical procedures, the complications can be very devastating, such as injury to the brain stem, spinal cord or vertebral arteries. It has also been found that optimum plating systems permit the placement of more than one screw in each of the instrumented vertebrae.
Many spinal plating systems have been developed in the last couple of decades that address some of the needs and requirements for cervical fixation systems. However, even with the more refined plating system designs, there still remains a need for a system that has universal applicability to all pathologies that can be faced by a spinal surgeon in treating the spine. For example, it has been demonstrated that different degrees of fixation of a bone screw relative to the plate are more advantageous for treating certain pathologies as opposed to other pathologies.
More specifically, it is known that bone screws can be supported in a spinal plate in either a rigid or a semi-rigid fashion. In a rigid fashion, the bone screws are not permitted any micro-motion or angular movement relative to the plate. In the case of a semi-rigid fixation, the bone screw can move somewhat relative to the plate during the healing process of the spine. It has been suggested that semi-rigid fixation is preferable for the treatment of degenerative diseases of the spine. In cases where a graft is implanted to replace the diseased vertebral body, the presence of a screw capable of some rotatation ensures continual loading of the graft. This continual loading avoids stress shielding of the graft, which in turn increases the rate of fusion and incorporation of the graft into the spine.
Similarly, rigid screw fixation is believed to be preferable in the treatment of tumors or trauma to the spine, particularly in the cervical region. It is believed that tumor and trauma conditions are better treated in this way because the rigid placement of the bone screws preserves the neuro-vascular space and provides for immediate stabilization. It can certainly be appreciated in the case of a burst fracture or large tumorous destruction of a vertebra that immediate stabilization and preservation of the disc space and neuro space is essential. On the other hand, the semi-rigid fixation is preferable for degenerative diseases because this type of fixation allows for a dynamic construct. In degenerative conditions, a bone graft is universally utilized to maintain either the disc space and/or the vertebral body itself. In most cases, the graft will settle or be at least partially resorbed into the adjacent bone. A dynamic construct, such as that provided by semi-rigid bone screw fixation, will compensate for this phenomenon.
At present, no plating system is known that allows for the placement of bone screws in either a semi-rigid or a rigid construct with a single plate. While some plates will provide for either one of these screw fixations, no plating system allows the surgeon to use a single plate and to select which of the two types of fixation is desired for the particular spinal pathology and anatomy.
In order to address the needs left unfulfilled by prior systems, the present invention contemplates a novel system for anterior fixation of the spine utilizing an elongated fixation plate. In one aspect of the invention, the fixation plate is provided with a plurality of holes through which bone screws extend for engagement into a number of vertebrae. In the preferred embodiment, the screw holes include a spherical portion to receive a complementary formed spherical head of the bone screw. The screw holes further include a cylindrical portion integral with the cylindrical portion and opening to the bottom of the plate.
The flexibility of the present invention anterior fixation plating system is accomplished by the provision of a fixed angle and a variable angle screw that can be supported within the same screw hole in the plate. Each of the screws includes a threaded shank for screwing into a vertebra and a spherical head to seat within the spherical recess. Both screws include an intermediate portion between the spherical head and the threaded shank that is configured to reside within the cylindrical portion of the screw hole when the screw is fixed to the plate. The intermediate portion of the fixed angle screw preferably is cylindrical and has an outer diameter sized for a close fit within the cylindrical portion of the screw hole. In this manner, the fixed angle screw is prevented from rotating or pivoting within the screw hole.
The variable angle bone screw includes an intermediate portion that is also preferably cylindrical. However, the cylindrical intermediate portion of the variable angle screw has an outer diameter that is significantly smaller than the diameter of the cylindrical portion of the screw hole. This relative difference in diameters between the screw intermediate portion and the screw hole allows the screw to assume a range of angles relative to the bottom surface of the plate, even when the screw is locked in position in the plate.
In a further aspect of the invention, a locking assembly is provided for locking the bone screws in the plate, thereby preventing screw back-out. In one embodiment of the invention, the locking assembly includes a washer that resides within a recess in the plate. The recess overlaps at least one screw hole in the plate so that the washer can be seated above the head of the bone screw to hold the screw in place. The locking assembly further includes a threaded set screw that is engaged within a tapered bore concentrically situated within the washer recess. The locking washer is itself configured with a recess so that the head of the locking screw can reside essentially flush within the locking washer.
In a further embodiment, the locking assembly includes a locking washer having cut-outs formed in its outer surface facing the bone screw head. The cut-outs preferably correspond to the shape of the screw hole so that the washer does not overlap the screw hole in the region of the cut-out. The washer is held to the plate by a set screw to permit rotation of the washer from a first position in which the cut-outs are aligned with the screw holes, to a second position in which the outer surface of the washer overlaps the screw holes to lock the screw heads in position. In a further aspect, the washer includes a number of keys projecting from its underside that are configured to mate within corresponding notches formed in the locking washer recess. The notch and key configuration essentially locks the washer in its position overlapping the screw holes.
The fixation plate assembly of the present invention further contemplates various screw hole arrangements that permit clamping of multiple screws by a single locking assembly. Other hole arrangements are provided that offer several screw fixation options to the surgeon when instrumenting one or more vertebrae. For instance, a four hole arrangement is set forth in which four screw holes a spaced at 90xc2x0 intervals around a single locking washer and screw assembly. The surgeon can insert either fixed or variable angle screws into any one or more of the four screw holes depending upon the vertebral anatomy.
It is one object of the invention to provide a fixation plate assembly that accommodates either fixed or variable angle screw fixation in a single plate. Another object resides in the provision of a locking assembly that can lock one or more bone screws within the plate.
One benefit achieved by the present invention is that the fixation plate and locking assembly maintain a low profile within the spine. A further benefit is accomplished by the ability to select either a fixed angle or a variable angle screw at any instrumented level and within a single fixation plate.
Other objects and benefits of the invention will become apparent upon consideration of the following written description of the invention, together with the accompanying figures.